The Kindle has been the best-selling item on Amazon.com for two years, the company says on its Web site, although it has never publicly stated its sales numbers.

The new versions have 20% faster page turns than the second-generation model, according to Amazon. They each weigh 8.7 ounces, which is 15% lighter than the previous version. They each have 4GB of storage, which is enough room for about 3,500 books and is double the storage space of previous models. They also have 6-inch diagonal screens, the same as the previous version.